A Ferrari F40 does almost exactly 200mph and "only" has 479 bhp so I would think an RS6 with the limiter removed might be able to reach 200mph with a stock engine. The drag coefficient is only around 0.01 more on the Audi too..
It has to be chipped and tuned. Stock they are about 600bph. To hit 200mph like that would need at least another 200 - They are restricted to 155mph otherwise. 180 would be about the limit without the limiter. There is so much aerodynamics to deal with, not to mention taking the tyres way beyond their design speed.
he drag coefficient is only around 0.01 more on the Audi too..
Weight affects acceleration not top speed, I agree to 4 wheel drive train would sap some power but the Ferrari has much wider rear tyres that would add to its parasitic losses so I would still think it may achieve 200mph with 600 bhpAlmost twice the weight though, I'd guess.
Edit; ferrari C. 1300kg
Audi. C. 2100kg
Plus, I'd guess the Audi has a greater mechanical drag, having to turn a heavy four wheel drivetrain?
I would still think it may achieve 200mph with 600 bhp